With many applications, semiconductor chips are mounted onto a metallic substrate, a so-called leadframe. In doing so, the leadframes are presented in magazines or in a stack from where they are removed by a robot and fed to a transport system of an automatic assembly machine, a so-called Die Bonder, which transports the leadframes one after the other to a dispensing station where adhesive or solder is applied and to a bonding station where the semiconductor chip is mounted.
When the leadframes are presented in a stack, it often happens that, on removal, two leadframes stick together so that instead of a single leadframe two leadframes are removed and passed on to the transport system. In order to detect such errors, the thickness is determined of the leadframe which is passed on to the transport system. This normally takes place in a mechanical way, as a rule by means of a sampling pin. When the measured thickness exceeds a predetermined maximum value this means that two leadframes have been removed. As a result, the robot is instructed to remove the leadframes from the transport system and to feed them to a trash container. This type of inspection has the disadvantage that the error is detected late. It takes time to remove two leadframes which are stuck together from the transport system and to supply a new leadframe, which reduces the throughput.